The invention disclosed in this specification relates to an inspection of comparing an image of a subject obtained by using light, laser light, or an electron beam with a reference image and detecting fine-pattern defects, foreign particles, etc. on the basis of a result of the comparison. In particular, the invention relates to a defect inspection method and apparatus which are suitable for an appearance inspection of semiconductor wafers, TFTs, photomasks, etc.
Among conventional techniques for detecting defects by comparing an inspection subject image with a reference image is a method disclosed in JP-A-5-264467 (Patent document 1).
In this technique, repetitive patterns that are arranged regularly on an inspection subject sample are shot sequentially and each resulting image is compared with an image that has been delayed by a time corresponding to a pattern repetition pitch. Non-coincident portions are detected as defects. This kind of conventional inspection method will be described below by taking, as an example, a defect inspection of a semiconductor wafer. As shown in FIG. 2(a), a number of chips having the same pattern are arranged regularly on a semiconductor wafer as an inspection subject. In memory devices such as DRAMs, each chip can be generally divided into memory mat portions 20-1 and a peripheral circuit portion 20-2. Each memory mat portion 20-1 is a set of small repetitive patterns (cells), and the peripheral circuit portion 20-2 is basically a set of random patterns. In general, in each memory mat portion 20-1, the pattern density is high and an image obtained is dark. On the other hand, in the peripheral circuit portion 20-2, the pattern density is low and an image obtained is bright.
In the conventional pattern inspection, for the peripheral circuit portion 20-2, images of regions located at the same position of adjoining chips are compared with each other; for example, regions 22 and 23 shown in FIG. 2(a) are compared with each other. A portion having a luminance difference that is larger than a threshold value is detected as a defect. In the following, this type of inspection will be referred to as “chip comparison.” For each memory mat portion 20-1, images of adjoining cells in the memory mat portion 20-1 are compared with each other. A portion having a luminance difference that is larger than a threshold value is likewise detected as a defect. In the following, this type of inspection will be referred to as “cell comparison.” These comparative inspections need to be performed at high speed.
JP-A-2001-5961 (Patent document 2) discloses a defect inspection apparatus which performs, in parallel, positional deviation detection and positional deviation correction and comparative image processing on multi-channel image signals received from an image sensor in parallel and multi-channel reference image signals obtained from a delay circuit section.
JP-A-2004-271470 (Patent document 3) discloses a pattern inspection apparatus which processes images at a processing speed that is approximately the same as an image capturing speed of an image sensor by performing, in the form of parallel processing, positional deviation correction, brightness correction, and defect detection on images taken by the image sensor and captured.
JP-A-2005-158780 (Patent document 4) discloses a pattern defect inspection apparatus in which pieces of image acquisition processing are performed in parallel for plural inspection areas on a sample by using plural image sensors and defects are detected by processing acquired images and classified asynchronously with the image acquisition.
JP-A-2005-321237 (Patent document 5) discloses a pattern inspection apparatus which is equipped with plural detection optical systems, plural image comparison processing means corresponding to the respective detection optical systems, and a classification processing means and which thereby detects a variety of detects with high sensitivity.
On the other hand, the invention disclosed in this specification relates to a defect inspection method and apparatus for inspecting a situation of occurrence of defects such as foreign particles in a manufacturing process. The defect inspection method and apparatus detect defects such as foreign particles occurring in a manufacturing process for producing a subject by forming patterns on a substrate such as a semiconductor manufacturing process, a liquid crystal display device manufacturing process, or a printed circuit board manufacturing process, and take a proper countermeasure by analyzing the defects.
In conventional semiconductor manufacturing processes, foreign particles existing on a semiconductor substrate (inspection subject substrate) may cause a failure such as an interconnection insulation failure or short-circuiting. If minute foreign particles exist on a semiconductor substrate bearing very fine semiconductor devices, the foreign particles may cause a capacitor insulation failure or breakage of a gate oxide film or the like. Such foreign particles exist in various states after being mixed in various manners; for example, they are generated from a movable portion of a transport apparatus or from human bodies, are generated through reaction involving a process gas in a processing apparatus, or are ones originally mixed in chemicals or materials.
Likewise, in conventional liquid crystal display device manufacturing processes, if a certain defect occurs because of a foreign particle placed on a pattern, the liquid crystal display device is rendered not suitable for use as a display device. The same is true of printed circuit board manufacturing processes. Mixing of foreign particles is a cause of pattern short-circuiting or a connection failure. One conventional technique for detecting such foreign particles on a semiconductor substrate is disclosed in JP-A-62-89336 (Conventional technique 1). In this technique, laser light is applied to a semiconductor substrate and scattered light which comes from foreign particles if they are attached to the semiconductor substrate is detected. A detection result is compared with one obtained immediately before for a semiconductor substrate of the same type. This prevents false judgments due to patterns and enables a high-sensitivity, high-reliability foreign particle/defect inspection. JP-A-63-135848 (Conventional technique 2) discloses a technique in which laser light is applied to a semiconductor substrate and scattered light which comes from foreign particles if they are attached to the semiconductor substrate is detected. The detected foreign particles are analyzed by laser photoluminescence, secondary X-ray analysis (XMR), or the like.
Among techniques for detecting foreign particles is a method which detects non-repetitive foreign particles or defects in an emphasized manner by illuminating an inspection subject substrate with coherent light and eliminating, with a spatial filter, light that is emitted from repetitive patterns on the inspection subject substrate.
JP-A-1-117024 (Conventional technique 3) discloses a foreign particle inspection apparatus in which light is applied to circuit patterns formed on an inspection subject substrate from a direction that is inclined by 45° from major straight lines of the circuit patterns, whereby 0th-order diffraction light is prevented from entering the opening of an objective lens. JP-A-117024 refers to interruption of light coming from other straight lines (which are not the major ones) with a spatial filter.
Conventional techniques relating to apparatus and methods for inspecting a subject for defects such as foreign particles are disclosed in JP-A-1-250847 (Conventional technique 4), JP-A-6-258239 (Conventional technique 5), JP-A-6-324003 (Conventional technique 6), JP-A-8-210989 (Conventional technique 7), and JP-A-8-271437 (Conventional technique 8).
JP-A-2006-145305 (Conventional technique 9) discloses a surface inspection apparatus which finds the thickness and the properties of a thin film formed on an inspection subject substrate by detecting plural polarization components simultaneously.
Among techniques for detecting plural polarization components simultaneously are polarimetry using channel spectra which is disclosed in Kazuhiko Oka, “Spectral Polarimetry Using Channel Spectra,” O plus E, Vol. 25, No. 11, p. 1,248, 2003 (Non-patent document 1), polarimetry using birefringent wedges which is disclosed in Non-patent document 1 and K. Oka, “Compact Complete Imaging Polarimeter Using Birefringent Wedge Prisms,” Optics Express, Vol. 11, No. 13, p. 1,510, 2003 (Non-patent document 2), and polarimetry using amplitude-division prisms and polarimetry using a minute polarizing element array which are disclosed in Hisao Kikuta et al., “Polarization Image Measuring System, O plus E, Vol. 25, No. 11, p. 1,241, 2003 (Non-patent document 3).